Photosensor, semiconductor device including photosensor, and light measurement method using photosensor

ABSTRACT

An object is to provide a photosensor utilizing an oxide semiconductor in which a refreshing operation is unnecessary, a semiconductor device provided with the photosensor, and a light measurement method utilizing the photosensor. It is found that a constant gate current can be obtained by applying a gate voltage in a pulsed manner to a transistor including a channel formed using an oxide semiconductor, and this is applied to a photosensor. Since a refreshing operation of the photosensor is unnecessary, it is possible to measure the illuminance of light with small power consumption through a high-speed and easy measurement procedure. A transistor utilizing an oxide semiconductor having a relatively high mobility, a small S value, and a small off-state current can form a photosensor; therefore, a multifunction semiconductor device can be obtained through a small number of steps.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensor, and an embodiment of theinvention disclosed herein relates to a photosensor utilizing an oxidesemiconductor and a semiconductor device including the photosensor.Further, an embodiment of the present invention relates to a lightmeasurement method using the photosensor.

2. Description of the Related Art

A number of detectors used for detecting an electromagnetic wave areknown, and for example, detectors having sensitivity to light from theultraviolet range to the infrared range are collectively referred to asphotosensors. Among them, a photosensor having sensitivity to light in avisible light range from a wavelength of 400 nm to a wavelength of 700nm is particularly referred to as a visible light sensor, and manyvisible light sensors are used for devices which need luminanceadjustment, on/off control, or the like depending on the usageenvironment.

Some display devices detect ambient brightness of a display portion toadjust the luminance of the display. This is because wasted electricpower of the display device can be reduced by increasing visibilitythrough detecting ambient brightness with the use of a photosensor andperforming display with an appropriate luminance. For example, asexamples of the display device including a photosensor for adjusting theluminance, mobile phones, computers, and the like can be given. Further,as well as the ambient brightness of the display portion, luminance ofthe backlight of a display device, in particular, a liquid crystaldisplay device is also detected by a photosensor to adjust the luminanceof a display screen.

A photosensor, for example, includes a photoelectric conversion elementsuch as a photodiode in a light sensing portion and can detectilluminance based on the amount of current which flows to thephotoelectric conversion element. In Patent Document 1, a chargeaccumulation type photosensor is disclosed. This photosensor measuresilluminance by utilizing such a property that the amount of electriccharge flowing through a photodiode varies in accordance with theilluminance. More specifically, the illuminance is measured in thefollowing manner: after electric charge is accumulated in a condenser(capacitor), a change in potential caused by discharge of electricity bya constant current circuit (a constant current power supply) is detectedby a comparator, the time needed for the change in potential isconverted to a digital signal by a counter circuit and a latch circuit,and then the digital signal is output.

As a photosensor, a sensor in which characteristics are changed by lightcan be utilized. For example, it is well known that conductivity of anoxide semiconductor changes by reception of light, and this fact isdescribed in Patent Document 2 and Patent Document 3. In thesedocuments, a technique for controlling the threshold of a transistorincluding an oxide semiconductor is disclosed. Further, Patent Document4 discloses an attempt to use an oxide semiconductor in a photosensor ora memory.

[Reference] [Patent Document]

[Patent Document 1] Japanese Published Patent Application No. H6-313840

[Patent Document 2] Japanese Published Patent Application No.2009-111125

[Patent Document 3] Japanese Published Patent Application No.2009-212443

[Patent Document 4] Japanese Published Patent Application No.2009-182194

SUMMARY OF THE INVENTION

Oxide semiconductors have advantages in having a higher mobility thanamorphous silicon and, in addition, being able to be entirely formedover a several-meter-square substrate called “G10” as a uniform film.These advantages enable a large-sized flat panel display capable of highspeed operation to be realized, for example. If a photosensor includingan oxide semiconductor can be manufactured, the photosensor and atransistor can be manufactured using the same oxide semiconductor layer,enabling a semiconductor device including a photosensor, for example, alarge-sized multifunction flat panel display to be manufactured in asmall number of steps. Although the photosensor including an oxidesemiconductor is disclosed in Patent Document 4, in this photosensor,characteristics changed by reception of light do not return to anoriginal state unless any operation is performed, and a refreshingoperation needs to be performed for every measurement. It is preferablefor a photosensor to measure illuminance on a several second cycle;however, if the refreshing operation is performed on a several secondcycle, significant power is consumed, which is not preferable.Therefore, an object of an embodiment of the present invention is toprovide a method for measuring light with the use of a transistor whichincludes a channel including an oxide semiconductor, in which arefreshing operation is unnecessary. Further, another object of anembodiment of the present invention is to provide a photosensor havingsuch a feature and a semiconductor device provided with the photosensor.

An embodiment of the present invention is a light measurement methodincluding the steps of applying a negative gate voltage to a transistorwhich includes a channel including an oxide semiconductor in a pulsedmanner, and measuring an illuminance of light received by the channelfrom an obtained gate current.

The applied gate voltage is preferably higher than or equal to −10 V andlower than or equal to −2 V. At this time, the voltage applied to asource electrode and a drain electrode of the transistor is set to 0,that is, grounded.

A similar effect can be obtained when the gate voltage is set to 0, thatis, grounded and a voltage of higher than or equal to 2 V and lower thanor equal to 10 V is applied to the source electrode and the drainelectrode of the transistor.

The gate voltage is applied for a period of more than or equal to 0.01ms and less than or equal to 100 ms, preferably more than or equal to 1ms and less than or equal to 2 ms. In the case where the applicationtime is more than 100 ms, gate current is significantly changed in thatperiod and the measurement accuracy is lowered. In the case where theapplication time is less than 0.01 ms, a high-performance circuit isneeded, which is not preferable. Thus, an application time of more thanor equal to 1 ms and less than or equal to 2 ms is preferable in termsof high measurement accuracy.

The number of applications of the gate voltage per unit time ispreferably more than or equal to 30 times per minute and less than orequal to 60 times per minute. With this number of applications, lightcan be measured with such a frequency as to enable adjustment of aluminance or the like so that a person can perceive light as beingcontinuous under a normal environment. However, the number ofapplications is not limited to this range and may be selected asappropriate by a practitioner in accordance with the usage. The numberof applications can also be called a frequency if the applications areconstant; however, it is not always necessary to apply a voltageperiodically.

Another embodiment of the present invention is a photosensor whichincludes a transistor including a channel including an oxidesemiconductor and an oscillator circuit, in which an output of theoscillator circuit is electrically connected to a gate electrode of thetransistor and the channel is a light receiving portion.

Another embodiment of the present invention is a photosensor whichincludes a transistor including a channel including an oxidesemiconductor and an oscillator circuit, in which an output of theoscillator circuit is electrically connected to a source electrode and adrain electrode of the transistor and the channel is a light receivingportion.

Another embodiment of the present invention is a semiconductor deviceincluding the above-described photosensor and an RFID device thatoperates using a transistor including a channel including the samematerial as the channel of the transistor included in the photosensor.

Another embodiment of the present invention is a semiconductor deviceincluding the above-described photosensor and a display device thatoperates using a transistor including a channel including the samematerial as the channel of the transistor included in the photosensor.

Another embodiment of the present invention is a semiconductor deviceincluding the above-described photosensor and electronic paper thatoperates using a transistor including a channel including the samematerial as the channel of the transistor included in the photosensor.

Note that in this specification, an oscillator circuit refers to the onethat applies a voltage as a gate voltage in a pulsed manner; a ringoscillator may be used for example. Further, a divider circuit may alsobe added in order to control the application time and the like. Furtherin this specification, “electrical connection” also includes wirelessconnection.

A transistor utilizing an oxide semiconductor having a relatively highmobility, a small S value, and a small off-state current can form aphotosensor according to an embodiment of the present invention;therefore, a multifunction semiconductor device can be obtained througha small number of steps. As examples of such a semiconductor device,there are flat panel displays that are capable of double-frame ratedriving or quadruple-frame rate driving utilizing a high mobility andprovided with a photosensor, RFID (radio frequency identification)devices provided with a photosensor, and the like. Since the refreshingoperation of the photosensor is unnecessary in the case where thevoltage is applied in a pulsed manner, it is possible to measure theilluminance of light with small power consumption through a high-speedand easy measurement procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E illustrate an example of a manufacturing process of aphotosensor according to an embodiment of the present invention;

FIGS. 2A to 2D illustrate an example of a manufacturing process of aphotosensor according to an embodiment of the present invention;

FIGS. 3A and 3B illustrate a relation between a gate current and a gatevoltage;

FIGS. 4A and 4B illustrate a relation between a gate current and a gatevoltage;

FIGS. 5A and 5B illustrate a relation between a gate current and a gatevoltage;

FIGS. 6A to 6C are cross-sectional views each illustrating a photosensoraccording to an embodiment of the present invention;

FIG. 7 is a circuit diagram of a photosensor according to an embodimentof the present invention;

FIGS. 8A to 8C are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIGS. 9A to 9C are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIG. 10 is a plan view of a semiconductor device;

FIGS. 11A1, 11A2, and 11B illustrate semiconductor devices;

FIG. 12 illustrates a semiconductor device;

FIG. 13 illustrates a semiconductor device;

FIGS. 14A to 14C illustrate semiconductor devices;

FIGS. 15A and 15B illustrate a semiconductor device;

FIGS. 16A and 16B illustrate examples of a usage mode of electronicpaper;

FIG. 17 is an external view of an example of an electronic book reader;

FIGS. 18A and 18B are external views illustrating an example of atelevision device and an example of a digital photo frame;

FIGS. 19A and 19B are external views each illustrating an example of anamusement machine;

FIGS. 20A and 20B are external views each illustrating an example of amobile phone, and

FIG. 21 illustrates an example of a structure of an RFID tag.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments are described in detail using the drawings.Note that the present invention is not limited to the description of theembodiments, and it is apparent to those skilled in the art that modesand details can be modified in various ways without departing from thespirit of the present invention disclosed in this specification and thelike. Structures of different embodiments can be implemented incombination as appropriate. Note that in the structure of the inventiondescribed below, identical components or components having similarfunctions are denoted by the same reference numerals, and thedescription thereof is not repeated. In addition, the semiconductordevice in this specification indicates all devices that operate byutilizing semiconductor characteristics.

Embodiment 1

In this embodiment, an example of a manufacturing method of aphotosensor utilizing a transistor having a bottom-gate structureincluding an oxide semiconductor and an example of a method formeasuring illuminance with the use of the photosensor will be describedwith reference to drawings. Alternatively, the transistor may have atop-gate structure. The photosensor is one kind of semiconductorelements.

First, a conductive layer 102 is formed over a substrate 100 (see FIG.1A).

Any substrate having an insulating surface can be used as the substrate100 and, for example, a glass substrate can be used. Further, it ispreferable that the glass substrate be a non-alkali glass substrate. Asa material of the non-alkali glass substrate, a glass material such asaluminosilicate glass, aluminoborosilicate glass, barium borosilicateglass, or the like is used, for example. Besides, as the substrate 100,an insulating substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate; a semiconductorsubstrate formed of a semiconductor material such as silicon, whosesurface is covered with an insulating material; a conductive substrateformed of a conductive material such as metal or stainless steel, whosesurface is covered with an insulating material can be used. In addition,a plastic substrate can be used as long as it can withstand heattreatment in a manufacturing process.

The conductive layer 102 is preferably formed using a conductivematerial such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten(W), or titanium (Ti). As a formation method, a sputtering method, avacuum evaporation method, a plasma CVD method, and the like are given.In the case of using aluminum (or copper) for the conductive layer 102,since aluminum itself (or copper itself) has disadvantages such as lowheat resistance and a tendency to be corroded, it is preferablydeposited in combination with a conductive material having heatresistance.

As the conductive material having heat resistance, it is possible to usea metal containing an element selected from titanium (Ti), tantalum(Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), andscandium (Sc), an alloy containing any of these elements as a component,an alloy containing a combination of any of these elements, a nitridecontaining any of these elements as a component, or the like. Theconductive layer 102 may be formed by stacking the conductive materialhaving heat resistance and aluminum (or copper).

Although not shown, the substrate 100 may be provided with a base layer.The base layer has a function of preventing diffusion of an impurityfrom the substrate 100, such as an alkali metal (Li, Cs, Na, or thelike), an alkaline earth metal (Ca, Mg, or the like), or the like. Inother words, provision of the base layer can realize improvement in thereliability of the semiconductor device. The base layer may be formed tohave a single-layer structure or a stacked structure using a variety ofinsulating materials such as silicon nitride or silicon oxide.Specifically, for example, a structure in which silicon nitride andsilicon oxide are stacked in that order over the substrate 100 isfavorable. This is because silicon nitride has a high blocking effectagainst an impurity. At the same time, in the case where silicon nitrideis in contact with a semiconductor, there is a possibility that aproblem occurs in the semiconductor element; thus, silicon oxide ispreferably applied as a material in contact with the semiconductor.

Next, a resist mask 104 is selectively formed over the conductive layer102 and the conductive layer 102 is selectively etched using the resistmask 104, whereby a conductive layer 106 which functions as a gateelectrode is formed (see FIG. 1B).

The resist mask 104 is formed through steps such as application of aresist material, exposure to light using a photomask, and development.For the application of the resist material, a method such as a spincoating method can be employed. Alternatively, the resist mask 104 maybe selectively formed by a droplet discharge method, a screen printingmethod, or the like. In this case, the steps of exposure to light usinga photomask, development, and the like are not needed; therefore,improvement in productivity can be achieved. Note that the resist mask104 is removed after the conductive layer 106 is formed by etching theconductive layer 102.

For the above etching, dry etching or wet etching may be used. In orderto improve coverage with a gate insulating layer or the like which isformed later and prevent disconnection of such a layer, the etching ispreferably performed so that end portions of the conductive layer 106are tapered. For example, the end portions are preferably tapered tohave a taper angle of more than or equal to 20° and less than 90°. Here,the “taper angle” refers to an acute angle formed by a side surface of alayer having a tapered shape and a bottom surface of the layer when across section of the layer is observed.

Next, an insulating layer 108 which functions as a gate insulating layeris formed so as to cover the conductive layer 106 (see FIG. 1C). Theinsulating layer 108 is formed using a material such as silicon oxide,silicon oxynitride, silicon nitride, silicon nitride oxide, aluminumoxide, or tantalum oxide, for example. Alternatively, the insulatinglayer 108 may be formed of stacked layers of these materials. Thethickness of the insulating layer 108 is preferably greater than orequal to 5 nm and less than or equal to 250 nm. For example, siliconoxide with a thickness of 100 nm is formed by a sputtering method. Aninsulating layer formed by a sputtering method contains a small amountof hydrogen and nitrogen and is preferable as a gate insulating layer.Although the effect of hydrogen, nitrogen, or the like in the film needsto be taken into consideration in the case where the insulating layer108 is formed using another method (such as a plasma CVD method), themethod for forming the insulating layer 108 is not particularly limitedas long as the desired insulating layer 108 can be obtained. Forexample, the insulating layer 108 is formed so as to include hydrogen ornitrogen at a concentration lower than that in an oxide semiconductorlayer to be formed later. More specifically, it is preferable that thehydrogen concentration in the insulating layer 108 be 1×10²¹ atoms/cm³or lower (further preferably 5×10²⁰ atoms/cm³ or lower); the nitrogenconcentration in the insulating layer 108 be 1×10¹⁹ atoms/cm³ or lower.Note that in order to obtain the insulating layer 108 having favorablecharacteristics, the temperature of the film formation is preferably400° C. or lower; however, an embodiment of the invention disclosedherein is not limited to this. Further, the above-describedconcentrations mean average values in the insulating layer 108.

Alternatively, the insulating layer 108 having a stacked structure maybe formed using a combination of a sputtering method and a CVD method (aplasma CVD method or the like). For example, a lower layer of theinsulating layer 108 (a region in contact with the conductive layer 106)can be formed by a plasma CVD method and an upper layer of theinsulating layer 108 can be formed by a sputtering method. A plasma CVDmethod enables a film with favorable step coverage to be formed withease; therefore, it is suitable for a method for forming a film justabove the conductive layer 106 having a large step. A sputtering methodcan easily reduce the hydrogen concentration of a formed film;therefore, it is suitable for a method for forming a film in contactwith an oxide semiconductor layer that is easily adversely affected byhydrogen. Thus, diffusion of hydrogen in the insulating layer 108 to theoxide semiconductor layer can be suppressed. Hydrogen existing in theoxide semiconductor layer or in a vicinity thereof has a significantlylarge influence on semiconductor characteristics, and this way offorming the insulating layer 108 is effective.

In this specification, an oxynitride refers to a substance in which theamount in atomic percent of oxygen is larger than that of nitrogen, anda nitride oxide refers to a substance in which the amount in atomicpercent of nitrogen is larger than that of oxygen.

Next, an oxide semiconductor layer 110 is formed to cover the insulatinglayer 108 (see FIG. 1D). In this embodiment, the oxide semiconductorlayer 110 includes a metal oxide semiconductor material.

The oxide semiconductor layer includes at least one element selectedfrom In, Ga, Sn, and Zn. For example, an oxide of four metal elements,such as an In—Sn—Ga—Zn—O-based oxide semiconductor; an oxide of threemetal elements, such as an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxidesemiconductor; an oxide of two metal elements, such as an In—Zn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor,a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxidesemiconductor, or an In—Ga—O-based oxide semiconductor; or an oxide ofone metal element, such as an In—O-based oxide semiconductor, aSn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor canbe used. In addition, any of the above oxide semiconductors may containan element other than In, Ga, Sn, and Zn, for example, SiO₂.

For example, an In—Ga—Zn—O-based oxide semiconductor means an oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn), andthere is no limitation on the composition ratio thereof.

For the oxide semiconductor layer, a thin film expressed by a chemicalformula InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or moremetal elements selected from Zn, Ga, Al, Mn, and Co. For example, M canbe Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the case where an In—Zn—O-based material is used for the oxidesemiconductor, a target with the following composition ratio is used:the composition ratio of In:Zn is 50:1 to 1:2 in an atomic ratio(In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably 20:1 to 1:1 in anatomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), furtherpreferably 15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in amolar ratio). For example, a target used for the formation of anIn—Zn—O-based oxide semiconductor has the following atomic ratio:In:Zn:O is X: Y:Z, where Z>1.5X+Y.

The oxide semiconductor layer 110 can be formed by a sputtering methodusing an oxide semiconductor target including In, Ga, and Zn(In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can be performed under thefollowing conditions, for example: the distance between the substrate100 and the target is more than or equal to 30 mm and less than or equalto 500 mm; the pressure is higher than or equal to 0.1 Pa and lower thanor equal to 2.0 Pa; direct current (DC) power supply is higher than orequal to 0.25 kW and lower than or equal to 5.0 kW; the temperature ishigher than or equal to 20° C. and lower than or equal to 100° C.; theatmosphere is a rare gas atmosphere of argon or the like, an oxygenatmosphere, or a mixed atmosphere of a rare gas such as argon andoxygen. As the above sputtering method, an RF sputtering method in whicha high frequency power supply is used as a power supply for sputtering,a DC sputtering method in which a DC power supply is used, a pulsed DCsputtering method in which a DC bias is applied in a pulsed manner, orthe like can be employed.

In this embodiment, the case where the oxide semiconductor layer 110 isformed to have a single-layer structure is described; however, the oxidesemiconductor layer 110 may have a stacked structure. For example,instead of the above structure, an oxide semiconductor layer having acomposition similar to that of the oxide semiconductor layer 110(hereinafter called an “oxide semiconductor layer with normalconductivity”) can be formed over the insulating layer 108, and afterthat, an oxide semiconductor layer whose constituent elements aresimilar to those of the oxide semiconductor layer 110 and whosecomposition ratio is different from that of the oxide semiconductorlayer 110 (hereinafter called an “oxide semiconductor layer with highconductivity”) can be formed. In this case, the oxide semiconductorlayer with high conductivity is positioned between a source electrode(or a drain electrode) and the oxide semiconductor layer with normalconductivity, which can improve element characteristics.

The ratio of the flow rate of an oxygen gas to the flow rate of an argongas in film formation conditions of the oxide semiconductor layer withhigh conductivity be smaller than that in film formation conditions ofthe oxide semiconductor layer with normal conductivity. Morespecifically, the oxide semiconductor layer with high conductivity isformed in a rare gas (such as argon or helium) atmosphere or anatmosphere containing an oxygen gas at 10% or less and a rare gas at 90%or more. The oxide semiconductor layer with normal conductivity isformed in an oxygen atmosphere or an atmosphere in which the flow rateof an oxygen gas is 1 time or more that of a rare gas. In such a manner,two kinds of oxide semiconductor layers having different conductivitiescan be formed.

In the case where the oxide semiconductor layer 110 is formed withoutexposure to the air after formation of the insulating layer 108,particles or moisture can be prevented from attaching to an interfacebetween the insulating layer 108 and the oxide semiconductor layer 110.

Note that the oxide semiconductor layer 110 may have a thickness ofapproximately 5 nm to 200 nm.

Next, a resist mask 112 is selectively formed over the oxidesemiconductor layer 110 and the oxide semiconductor layer 110 isselectively etched using the resist mask 112, whereby an oxidesemiconductor layer 114 is formed (see FIG. 1E). Here, the resist mask112 can be formed in a manner similar to that of the resist mask 104.Note that the resist mask 112 is removed after the oxide semiconductorlayer 114 is formed by etching the oxide semiconductor layer 110.

Either wet etching or dry etching can be employed as the etching of theoxide semiconductor layer 110. Here, an unnecessary portion of the oxidesemiconductor layer 110 is removed by wet etching using a mixed solutionof acetic acid, nitric acid, and phosphoric acid, so that the oxidesemiconductor layer 114 is formed. Note that the etchant (the etchingsolution) used in the above wet etching may be any solution which canetch the oxide semiconductor layer 110, and is not limited to theabove-described solution.

In the case of employing dry etching, a gas containing a chlorine atom(e.g., chlorine (Cl₂), chlorine dioxide (ClO₂)) or a gas containing achlorine atom to which oxygen (O₂) is added may be used. By using a gasincluding a chlorine atom, etching selectivity of the oxidesemiconductor layer 110 with respect to the insulating layer can beeasily obtained.

As an etching apparatus used for the dry etching, an etching apparatususing a reactive ion etching method (an RIE method), or a dry etchingapparatus using a high-density plasma source such as an ECR (electroncyclotron resonance) source or an ICP (inductively coupled plasma)source can be used. Alternatively, a technique similar to the abovetechnique may be employed.

Next, a conductive layer 116 is formed so as to cover the insulatinglayer 108 and the oxide semiconductor layer 114 (see FIG. 2A). Theconductive layer 116 can be formed using a material and a method similarto those of the conductive layer 102. For example, the conductive layer116 can be formed with a single-layer structure of molybdenum ortitanium. Alternatively, the conductive layer 116 may be formed with astacked structure and, for example, a stacked structure of aluminum andtitanium can be employed. Further, the conductive layer 116 may have athree-layer structure in which titanium, aluminum, and titanium arestacked in this order. Alternatively, a three-layer structure in whichmolybdenum, aluminum, and molybdenum are stacked in this order may beused. As the aluminum used for these stacked structures, an aluminumfilm including neodymium (Al—Nd) may be used. Further alternatively, theconductive layer 116 may have a single-layer structure of an aluminumfilm containing silicon.

Next, a resist mask 118 and a resist mask 120 are selectively formedover the conductive layer 116 and the conductive layer 116 isselectively etched using the resist masks, so that a conductive layer122 which functions as one of source and drain electrodes and aconductive layer 124 which functions as the other of source and drainelectrodes are formed (see FIG. 2B). Here, the resist masks 118 and 120can be formed in a manner similar to that of the resist mask 104. Notethat the resist masks 118 and 120 are removed after the conductivelayers 122 and 124 are formed by etching the conductive layer 116.

The resist mask 118 and the resist mask 120 may be formed using amulti-tone mask. Here, the multi-tone mask is a mask capable of lightexposure with multi-level light intensity. With the use of a multi-tonemask, a one-time exposure and development process can form a resist maskwith plural thicknesses (typically, two kinds of thicknesses). By usingthe multi-tone mask, the number of steps can be reduced.

Either wet etching or dry etching can be employed as the etching of theconductive layer 116. Here, an unnecessary portion of the conductivelayer 116 is removed by dry etching, so that the conductive layer 122and the conductive layer 124 are formed.

Note that, although a structure (a channel-etch type) in which part ofthe oxide semiconductor layer 114 is removed when the conductive layer116 is etched is employed in this embodiment, an embodiment of theinvention disclosed herein is not limited to this. Instead, anotherstructure (an etching stopper type) can be employed in which a layer (anetching stopper) which prevents the etching from proceeding is formedbetween the oxide semiconductor layer 114 and the conductive layer 116so that the semiconductor layer 114 is not etched.

After the conductive layers 122 and 124 are formed, heat treatment isperformed at a temperature higher than or equal to 100° C. and lowerthan or equal to 500° C., typically higher than or equal to 200° C. andlower than or equal to 400° C. The atmosphere in which the heattreatment is performed can be, for example, an air atmosphere, anitrogen atmosphere, an oxygen atmosphere, or the like. Further, theheat treatment time can be approximately more than or equal to 0.1 hoursand less than or equal to 5 hours. Here, the heat treatment is performedat 350° C. in a nitrogen atmosphere for one hour. Note that the timingof the heat treatment is not particularly limited as long as it is afterthe oxide semiconductor layer 110 is formed and before an insulatinglayer serving as an interlayer insulating layer is formed. For example,the heat treatment may be performed just after the oxide semiconductorlayer 110 is formed. Alternatively, the heat treatment may be performedjust after the oxide semiconductor layer 114 is formed or just after theconductive layer 116 is formed. By performing the heat treatment (thefirst heat treatment) and the following heat treatment (the second heattreatment), characteristics of the semiconductor element can be improvedand variation in the characteristics can be suppressed.

Note that it is preferable that the above-described heat treatment beperformed at 400° C. or lower in order not to change (deteriorate)characteristics of the insulating layer 108 which functions as the gateinsulating layer. Needless to say, an embodiment of the inventiondisclosed herein should not be interpreted as being limited thereto.

The positional relation between the oxide semiconductor layer 114 andthe conductive layers 122 and 124 is not limited to the one illustratedin FIGS. 2A to 2D and the positions thereof may be inverted. An exampleof the structure in which the positions are inverted is illustrated in adrawing in a later embodiment of the invention. Further, a structure inwhich part of an oxide semiconductor layer is sandwiched betweenconductive layers or a structure in which part of a conductive layer issandwiched between oxide semiconductor layers may be employed. The samecan be said for a transistor having a top-gate structure.

Next, an insulating layer 126 is formed so as to cover the conductivelayer 122, the conductive layer 124, the oxide semiconductor layer 114,and the like (see FIG. 2C). Here, the insulating layer 126 serves as aso-called interlayer insulating layer. The insulating layer 126 can beformed using a material such as silicon oxide, aluminum oxide, ortantalum oxide. The insulating layer 126 may also be formed by stackingfilms of these materials.

The hydrogen concentration in the insulating layer 126 is, for example,lower than or equal to 1×10²¹ atoms/cm³ (preferably lower than or equalto 5×10²⁰ atoms/cm³). Further, the nitrogen concentration in theinsulating layer 126 is preferably lower than or equal to 1×10¹⁹atoms/cm³. Note that the above-described concentrations mean averagevalues in the insulating layer 126.

As a more specific example of the insulating layer 126 fulfilling theabove-described conditions, a silicon oxide formed by a sputteringmethod can be given. This is because, in the case of using a sputteringmethod, the hydrogen concentration in the film can be easily reduced.Needless to say, any of other methods including a plasma CVD method maybe employed as long as the above conditions are fulfilled. For example,after the insulating layer 126 is formed by a plasma CVD method, thehydrogen concentration in the insulating layer 126 can be reduced bysubjecting the insulating layer 126 to plasma treatment using a gasincluding a halogen element. The other conditions of the insulatinglayer 126 are not particularly limited. For example, the thickness ofthe insulating layer 126 can vary within a feasible range.

After that, a variety of electrodes and wirings are formed, whereby asemiconductor device provided with the transistor 150 is completed (seeFIG. 2D). In this embodiment, a typical example is shown in which aconductive layer 128 connected to the conductive layers 122 and 124functioning as a source and drain electrodes is formed. However, anembodiment of the invention disclosed herein is not limited to this.

After the conductive layer 128 is formed, heat treatment is performed ata temperature higher than or equal to 100° C. and lower than or equal to500° C., typically, higher than or equal to 200° C. and lower than orequal to 400° C. The atmosphere in which the heat treatment is performedcan be, for example, an air atmosphere, a nitrogen atmosphere, an oxygenatmosphere, or the like. Further, the heat treatment time can beapproximately more than or equal to 0.1 hours and less than or equal to5 hours. Here, the heat treatment is performed at 350° C. in a nitrogenatmosphere for one hour. Note that the timing of the heat treatment isnot particularly limited as long as it is after the formation of theinsulating layer 126. For example, the above heat treatment may beperformed just after the insulating layer 126 is formed. Alternatively,the above heat treatment may be performed after another insulatinglayer, another conductive layer, or the like is formed. By performingthe heat treatment (the second heat treatment) and the preceding heattreatment (the first heat treatment), characteristics of thesemiconductor element can be improved and variation in characteristicscan be suppressed.

Note that the effect of the second heat treatment is not limited to theabove. For example, the second heat treatment also provides anadvantageous effect of repairing defects in the insulating layer 126.Since the insulating layer 126 is formed at a relatively lowtemperature, the film includes defects. Accordingly, the elementcharacteristics might be adversely affected when the insulating layer126 is used as it is. From a perspective of repairing such defects inthe insulating layer 126, it can be said that the above-described heattreatment plays an important role.

In addition, it is preferable that the heat treatment be performed at400° C. or lower so as not to change (deteriorate) characteristics ofthe insulating layer 108 which functions as the gate insulating layer.Needless to say, an embodiment of the invention disclosed herein shouldnot be interpreted as being limited thereto.

Next, a method for measuring illuminance of light by utilizing thecompleted transistor 150 will be described. For example, first, light tobe measured enters the oxide semiconductor layer 114 from the upperdirection of the transistor 150. In this case, the insulating layer 126has to be the one that transmits the light to be measured.Alternatively, the substrate 100, the conductive layer 106, and theinsulating layer 108 may be formed to have a light-transmitting propertyto light to be measured so that the light to be measured enters theoxide semiconductor layer 114.

Then, gate current is measured under a state in which a negative gatevoltage is applied in a pulsed manner. In this case, the gate currentvaries depending on the illuminance of light, and by utilizing this, theilluminance can be measured. A specific example is described withreference to FIGS. 3A and 3B.

FIG. 3A shows an example of a relation between the applied gate voltageand the time. The vertical axis indicates the gate voltage Vg, and thehorizontal axis indicates the time t. In this example, the gate voltagewas −2V, the time for applying one pulse was 1 ms, and the pulsefrequency was 60 pulses per minute. The relation among the gate currentIg, the illuminance E of light entering the oxide semiconductor layer114, and the time t is shown in FIG. 3B. The left vertical axisindicates the absolute value of the gate current Ig, the right verticalaxis indicates the illuminance E of light, and the horizontal axisindicate the time t. When the illuminance of light to which thetransistor 150 was exposed is changed every 20 seconds, it was foundthat the gate current Ig obtained was constant corresponding to theilluminance. According to this experiment, at an illuminance of 0, theabsolute value of the gate current Ig was 4×10⁻¹² A (amperes); at anilluminance of 1, the absolute value of the gate current Ig was 1×10⁻¹⁰A (amperes); and at an illuminance of 2, the absolute value of the gatecurrent Ig was 8×10⁻¹⁰ A (amperes). Note that the illuminance of 0 meansa state in which the transistor 150 is irradiated with no light, theilluminance of 1 means a state in which the transistor 150 is irradiatedwith light having an illuminance that is not the illuminance of 0, andthe illuminance of 2 means a state in which the transistor 150 isirradiated with light having an illuminance higher than the illuminanceof 1. Thus, it was found that there is a positive correlation betweenthe illuminance E and the absolute value of the gate current Ig.Further, the reproducibility is high; therefore, it was found that thetransistor 150 can be used as a photosensor. Note that for applicationof a voltage in a pulsed manner, an oscillator circuit such as a ringoscillator may be used, for example. Further, a divider circuit may alsobe added in order to control the application time.

Next, it will be described why the gate voltage Vg has to be applied ina pulsed manner, along FIGS. 4A and 4B and FIGS. 5A and 5B.

FIGS. 4A and 4B show an example in which a constant gate voltage Vg (inthis example, Vg is −2 V) keeps being applied to the transistor 150which is not exposed to light, for 400 seconds. FIG. 4A shows a relationbetween the applied gate voltage Vg and the time t. At this time, thegate current Ig changes over time as shown in FIG. 4B. The vertical axisindicates the absolute value of the gate current Ig, and the horizontalaxis indicates the time t. In the graph, the absolute value of the gatecurrent Ig gradually decreases over the period of 400 seconds. Since thegate current Ig changes over time when the constant gate voltage Vg isapplied, it is difficult to measure the illuminance from the gatecurrent value.

On the other hand, FIGS. 5A and 5B show an example in which a gatevoltage Vg (in this example, Vg is −2 V) keeps being applied to thetransistor 150 which is not exposed to light in a pulsed manner, thatis, intermittently for 400 seconds. FIG. 5A shows a relation between theapplied gate voltage Vg and the time t. The time for applying one pulseof gate voltage was 1 ms, and the pulse frequency was 8 pulses perminute. At this time, the gate current Ig was almost constant as shownin FIG. 5B. The vertical axis indicates the absolute value of the gatecurrent Ig, and the horizontal axis indicates the time t. Theilluminance of light can be derived from the measured gate current Ig.

As described in this embodiment, the illuminance of light can bemeasured using the transistor including the oxide semiconductor layer114. A transistor utilizing an oxide semiconductor having a relativelyhigh mobility, a small S value, and a small off-state current can form aphotosensor according to an embodiment of the present invention;therefore, a multifunction semiconductor device can be obtained througha small number of steps. As examples of such a semiconductor device,there are flat panel displays that are capable of double-frame ratedriving or quadruple-frame rate driving utilizing a high mobility andprovided with a photosensor, RFID (radio frequency identification)devices provided with a photosensor, and the like. Since the refreshingoperation of the photosensor is unnecessary in the case where thevoltage is applied in a pulsed manner, it is possible to measure theilluminance of light with small power consumption through a high-speedand easy procedure.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 2

In this embodiment, an example of connecting a current amplifier to aphotosensor according to an embodiment of the present invention will bedescribed. Since the current detected by the photosensor is extremelyweak, this structure is preferable. Note that a photosensor providedwith a current amplifier may be collectively referred to as aphotosensor; however, a photosensor and a current amplifier areindividually named in this specification.

FIGS. 6A to 6C illustrate an example in which a photosensor and acurrent amplifier are both provided. Although a transistor including anoxide semiconductor can be used for the current amplifier, in such acase, a light-blocking portion is preferably provided becausecharacteristics of the transistor vary when the transistor is exposed tolight. Further, for a further increase of the current amplitude, aplurality of transistors may be provided in the current amplifier.

FIG. 6A illustrates an example in which two bottom-gate oxidesemiconductor transistors are provided. In the drawing, the lefttransistor is a current amplifier, and the right transistor is aphotosensor.

Next, components of each transistor are described. In the example ofFIG. 6A, the transistor of the current amplifier and the transistor ofthe photosensor have completely the same structure, and description willtherefore be made on only one of the two transistors here. A conductivelayer 602 functioning as a gate electrode is formed over a substrate601, and an insulating layer 603 functioning as a gate insulating layeris formed over the conductive layer 602. An insulating layer serving asa base layer may be provided between the substrate 601 and theconductive layer 602. Further, an oxide semiconductor layer 604 isprovided over the insulating layer 603, and a conductive layer 605functioning as a source electrode or a drain electrode is provided overthe oxide semiconductor layer 604. An insulating layer 606 covers theoxide semiconductor layer 604, the conductive layer 605, and the like,and an opening portion reaching the conductive layer 605 is provided inthe insulating layer 606. A conductive layer 607 functioning as a wiringfills the opening portion and electrically connects the transistors. Aninsulating layer 608 covers the conductive layer 607 and protects theconductive layer 607 from the outside, and an opening portion reachingpart of the conductive layer 607 is provided in the insulating layer608. A conductive layer 609 fills the opening portion and forms a wiringportion that is connected to a power supply line VDD or a power supplyline VSS provided outside. An opening portion 651 is provided in theconductive layer 609 in a region above the transistor serving as aphotosensor. Light enters through the opening portion 651, and theilluminance of the light can be measured. In addition, the conductivelayer 609 also has a role of shielding the transistor of the currentamplifier from light.

FIG. 6B illustrates an example in which two top-gate oxide semiconductortransistors are provided. In the drawing, the left transistor is acurrent amplifier, and the right transistor is a photosensor.

Next, components of each transistor are described. In the example ofFIG. 6B, the transistor of the current amplifier and the transistor ofthe photosensor have completely the same structure, and description willtherefore be made on only one of the two transistors here. Alight-blocking layer 610 is provided over part of a substrate 611 andshields the transistor of the current amplifier from light. Aninsulating layer 600 functioning as a base layer is provided between thesubstrate 611 and the light-blocking layer 610. In this manner, entry ofimpurities from the substrate 611 to the transistor can be prevented. Anoxide semiconductor layer 614 is provided over the insulating layer 600,and a conductive layer 615 functioning as a source electrode or a drainelectrode is provided over the oxide semiconductor layer 614. Aninsulating layer 613 covering the conductive layer 615 and the oxidesemiconductor layer 614 functions as a gate insulating layer. Further, aconductive layer 612 functioning as a gate electrode is provided overthe insulating layer 613, and an insulating layer 616 covers theconductive layer 612 and the insulating layer 613. An opening portionreaching the conductive layer 615 is provided in the insulating layer616 and the insulating layer 613. A conductive layer 617 functioning asa wiring fills the opening portion and electrically connects thetransistors and the like. An insulating layer 618 covers the conductivelayer 617 and protects the conductive layer 617 from the outside. Anopening portion reaching part of the conductive layer 617 is provided inthe insulating layer 618. A conductive layer 619 fills the openingportion and forms a wiring portion that is connected to a power supplyline VDD or a power supply line VSS provided outside. In the top-gatetransistor, light enters the oxide semiconductor layer 614 from thesubstrate 611 side, and the illuminance of the light is measured.Needless to say, depending on the kind of electrodes, light can entereither side of the transistor. In this example, owing to the existenceof the light-blocking layer 610, light enters only the transistor on theright side in the drawing.

FIG. 6C illustrates an example in which a top-gate oxide semiconductortransistor and a bottom-gate oxide semiconductor transistor areprovided. In the drawing, the left bottom-gate transistor a is a currentamplifier and the right top-gate transistor b is a photosensor; however,they may be interchanged.

Next, components of each transistor are described. A conductive layer622 a included in the transistor a and functioning as a gate electrodeis provided over a substrate 621. An insulating layer 630 functioning asa gate insulating layer of the transistor a is provided over theconductive layer 622 a. The insulating layer 630 also functions as abase layer of the transistor b. Further, an oxide semiconductor layer624 is provided over the insulating layer 630 and functions as a channelin both of the transistors. A conductive layer 625 functioning as asource electrode or a drain electrode is provided in contact with theoxide semiconductor layer 624. An insulating layer 623 functioning as agate insulating layer of the transistor b covers the oxide semiconductorlayer 624, the conductive layer 625, and the like. A conductive layer622 b functioning as a gate electrode of the transistor b is providedover the insulating layer 623, and an insulating layer 626 is providedso as to cover the conductive layer 622 b and the insulating layer 623.An opening portion reaching the conductive layer 625 is provided in theinsulating layer 626 and the insulating layer 623. A conductive layer627 functioning as a wiring fills the opening portion and electricallyconnects the transistors and the like. An insulating layer 628 coversthe conductive layer 627 and protects the conductive layer 627 from theoutside. An opening portion reaching part of the conductive layer 627 isprovided in the insulating layer 628. A conductive layer 629 fills theopening portion and forms a wiring portion that is connected to a powersupply line VDD or a power supply line VSS provided outside. In thetop-gate transistor, light enters the oxide semiconductor layer 624 fromthe substrate 621 side, and the illuminance of the light is measured. Atthis time, the conductive layer 622 a functioning as a gate electrode ofthe transistor a also functions as a light-blocking layer.

Next, a current amplification method will be described with reference tothe circuit diagram of FIG. 7. A photosensor 201 includes one transistor1 b. This transistor 1 b is connected to a current amplifier 200including N number of transistors ka (k is a natural number greater thanor equal to 1 and less than or equal to N), and a gate current Ig isamplified. The current amplifier 200 forms a so-called current mirrorcircuit. The gate current Ig is amplified by the N number of transistorsin the current amplifier 200 to become N times as large as the gatecurrent Ig. Accordingly, as N is larger, the current is amplified more.The transistor 1 b may be the above-described transistor including anoxide semiconductor, and a plurality of such transistors may be providedin the photosensor 201. The transistor ka may be the above-describedtransistor including the light-blocking layer, or may be a transistorthat does not include an oxide semiconductor.

Next, an example of measuring the illuminance of light in the circuitillustrated in FIG. 7 will be described. First, 0 V is applied to asource electrode and a drain electrode of the transistor 1 b, and −2 Vis applied to a gate electrode of the transistor lb. The applicationtime was more than or equal to 1 ms and less than or equal to 2 ms, andthe application frequency was more than or equal to 30 pulses per minuteand less than or equal to 60 pulses per minute. However, the applicationtime and the application frequency are not limited to these ranges andmay be determined as appropriate by a practitioner. Then, a gate currentIg having a constant value flows into the transistor 1 a, and a currentIg having the same amount as the current flowing into the transistor 1 aflows into the transistor ka (k is a natural number greater than orequal to 2 and less than or equal to N). In this manner, a current Ntimes as large as the current Ig can be obtained.

As described in this embodiment, the illuminance of light can bemeasured more accurately by using the combination of the currentamplifier 200 and the photosensor 201 including the transistor includingthe oxide semiconductor. A transistor utilizing an oxide semiconductorhaving a relatively high mobility, a small S value, and a smalloff-state current can form a photosensor according to an embodiment ofthe present invention; therefore, a multifunction semiconductor devicecan be obtained through a small number of steps. As examples of such asemiconductor device, there are flat panel displays that are capable ofdouble-frame rate driving or quadruple-frame rate driving utilizing ahigh mobility and provided with a photosensor, RFID (radio frequencyidentification) devices provided with a photosensor, and the like. Sincethe refreshing operation of the photosensor is unnecessary in the casewhere the voltage is applied in a pulsed manner, it is possible tomeasure the illuminance of light with small power consumption through ahigh-speed and easy procedure.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 3

In this embodiment, an RFID tag which is an example to which thetransistor including the oxide semiconductor described in the aboveembodiment is applied will be described with reference to FIG. 21.

The RFID tag of this embodiment includes a memory circuit storingnecessary data and exchanges data with the outside using contactlessmeans such as wireless communication. Having these features, the RFIDtag can be used for an individual authentication system in which anobject is identified by reading individual information of the object, orthe like. Note that the RFID tag is required to have extremely highreliability in order to be used for this purpose.

A configuration of the RFID tag will be described with reference to FIG.21. FIG. 21 is a block diagram illustrating a configuration example ofan RFID tag.

As shown in FIG. 21, an RFID tag 500 includes an antenna 504 whichreceives a radio signal 503 that is transmitted from an antenna 502connected to a communication device 501 (also referred to as aninterrogator, a reader/writer, or the like). The RFID tag 500 includes arectifier circuit 505, a constant voltage circuit 506, a demodulationcircuit 507, a modulation circuit 508, a logic circuit 509, a memorycircuit 510, and a ROM 511. In an embodiment of the present invention, aphotosensor 513 and an A/D converter 512 are added to the RFID tag 500.A transistor having a rectifying function included in the demodulationcircuit 507 preferably includes a material which enables a reversecurrent to be small enough, for example, an oxide semiconductor. Thiscan suppress the phenomenon of a rectifying function becoming weaker dueto generation of a reverse current and prevent saturation of the outputfrom the demodulation circuit. In other words, the input to thedemodulation circuit and the output from the demodulation circuit canhave a relation closer to a linear relation. Note that data transmissionmethods are roughly classified into the following three methods: anelectromagnetic coupling method in which a pair of coils is provided soas to face each other and communicates with each other by mutualinduction, an electromagnetic induction method in which communication isperformed using an induction field, and a radio wave method in whichcommunication is performed using a radio wave. Any of these methods canbe used in the RFID tag 500 of this embodiment.

Next, the structure of each circuit will be described. The antenna 504exchanges the radio signal 503 with the antenna 502 which is connectedto the communication device 501. The rectifier circuit 505 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 504 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 505.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 505. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 506 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 506 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 509 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 507 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 508 performs modulation in accordance with data to beoutput from the antenna 504.

The logic circuit 509 analyzes and processes the demodulated signal. Thememory circuit 510 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 511stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that any of the above-described circuits may be omitted asappropriate.

In this embodiment, the photosensor 513 according to an embodiment ofthe present invention is provided in the RFID tag 500. Therefore, thememory circuit 510 can record data about light such as the amount oflight the RFID tag 500 receives. For example, the RFID tag 500 is set ina cultivated field and keeps receiving a signal from the communicationdevice 501, so that the yearly amount of sunshine or the like can berecorded. In this case, since a voltage is applied in a pulsed manner tothe photosensor 513, an oscillator circuit is preferably provided in thecommunication device 501. This data is useful for a producer toinvestigate a relation between the amount of sunshine in the cultivatedfield and the quality of a harvest and the like and serves as acriterion for a consumer to decide to buy the harvest.

At least part of the demodulation circuit 507 and the photosensor can bemanufactured in the same process, a multifunction semiconductor devicecan be obtained through a small number of steps. In this specification,a channel of a transistor forming part of the demodulation circuit 507and a channel of the transistor forming part of the photosensor 513 areregarded as including the same material because these channels areformed through the same process. Further, since the refreshing operationof the photosensor is unnecessary in the case where the voltage isapplied in a pulsed manner, it is possible to measure the illuminance oflight with small power consumption through a high-speed and easyprocedure.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 4

In this embodiment, a manufacturing process of an active matrixsubstrate which is an example of a semiconductor device will bedescribed with reference to drawings. Note that the manufacturingprocess described in this embodiment and the manufacturing processdescribed in the previous embodiment have many points in common.Therefore, in the following description, repeated description of thesame portions is omitted, and different points are described in detail.Note that in the following description, FIGS. 8A to 8C and FIGS. 9A to9C are cross-sectional views and FIG. 10 is a plan view. In addition,line A1-A2 and line B1-B2 in each of FIGS. 8A to 8C and FIGS. 9A to 9Ccorrespond to line A1-A2 and line B1-B2 in FIG. 10, respectively. Notethat in this embodiment, a semiconductor element illustrated in a regionalong line A1-A2 is a bottom-gate transistor.

First, a wiring and an electrode (a gate electrode 302, a capacitorwiring 304, and a first terminal 306) are formed over a substrate 300(see FIG. 8A). Specifically, after a conductive layer is formed over thesubstrate, the wiring and electrode are formed through etching using aresist mask. In this embodiment, the wiring and electrode can be formedby a method similar to the method described in the previous embodiment;therefore, the previous embodiment (e.g., description with reference toFIGS. 1A and 1B) can be referred to for the details. Note that in theabove description, the distinction between “an electrode” and “a wiring”is made only for convenience, and their functions are not limited by thedenomination of “the electrode” or “the wiring”. For instance, a gateelectrode may refer to a gate wiring in some cases.

Note that the capacitor wiring 304 and the first terminal 306 can beformed at the same time using a material and a manufacturing methodwhich are the same as those of the gate electrode 302. Therefore, forexample, the gate electrode 302 and the first terminal 306 can beelectrically connected to each other. The previous embodiment can bereferred to for the details of the material and the manufacturing methodof the gate electrode 302.

Next, a gate insulating layer 308 is formed over the gate electrode 302and the gate insulating layer 308 is selectively etched so as to exposethe first terminal 306, whereby a contact hole is formed (see FIG. 8B).The previous embodiments (e.g., description with reference to FIG. 1C)can be referred to for the detail of the gate insulating layer 308.There is no particular limitation on the etching treatment; dry etchingmay be used, or wet etching may be used.

Next, after a conductive layer covering the gate insulating layer 308and the first terminal 306 is formed, the conductive layer isselectively etched, so that a source electrode 310 (or a drainelectrode), a drain electrode 312 (or a source electrode), a connectionelectrode 314, and a second terminal 316 are formed (see FIG. 8C). Notethat in the above description, the distinction between “an electrode”and “a wiring” is made only for convenience, and their functions are notlimited by the denomination of “the electrode” or “the wiring”. Forinstance, a source electrode may refer to a source wiring in some cases.

The previous embodiment (e.g., description with reference to FIGS. 2Aand 2B) can be referred to for the material, the manufacturing method,the etching treatment, or the like of the above-described conductivelayer. Note that by performing dry etching in the etching treatment, awiring structure can be miniaturized as compared with the case of usingwet etching. The connection electrode 314 can be directly connected tothe first terminal 306 through the contact hole formed in the gateinsulating layer 308. Note also that the second terminal 316 can beelectrically connected to the source electrode 310.

Next, after an oxide semiconductor layer is formed so as to cover atleast the source electrode 310 and the drain electrode 312, the oxidesemiconductor layer is selectively etched to form an oxide semiconductorlayer 318 (see FIG. 9A). Here, the oxide semiconductor layer 318 is incontact with parts of the source electrode 310 and the drain electrode312. The previous embodiment (e.g., description with reference to FIGS.1D and 1E) can be referred to for the detail of the oxide semiconductorlayer 318.

After the oxide semiconductor layer 318 is formed, heat treatment at atemperature higher than or equal to 100° C. and lower than or equal to500° C., typically higher than or equal to 200° C. and lower than orequal to 400° C., is performed. The atmosphere in which the heattreatment is performed can be, for example, an air atmosphere, anitrogen atmosphere, an oxygen atmosphere, or the like. Further, theheat treatment time can be approximately more than or equal to 0.1 hoursand less than or equal to 5 hours. Here, the heat treatment is performedat 350° C. in an air atmosphere for one hour. Note that the timing ofthe heat treatment is not particularly limited as long as it is afterthe oxide semiconductor layer 318 is formed and before an insulatinglayer serving as an interlayer insulating layer is formed. For example,the heat treatment may be performed just after the oxide semiconductorlayer 318 is formed. By performing the heat treatment (the first heattreatment) and the following heat treatment (the second heat treatment),characteristics of the semiconductor element can be improved andvariation in characteristics can be suppressed.

Note that it is preferable that the heat treatment be performed at 400°C. or lower so as not to change (deteriorate) characteristics of thegate insulating layer 308. Needless to say, an embodiment of theinvention disclosed herein should not be interpreted as being limitedthereto.

Then, an insulating layer 320 is formed so as to cover the sourceelectrode 310, the drain electrode 312, the oxide semiconductor layer318, and the like, and the insulating layer 320 is selectively etched soas to form contact holes which reach the drain electrode 312, theconnection electrode 314, and the second terminal 316 (see FIG. 9B). Theinsulating layer 320 is formed using a material such as silicon oxide,aluminum oxide, or tantalum oxide. The insulating layer 320 may also beformed by stacking films formed of these materials.

The hydrogen concentration in the insulating layer 320 is preferably1×10²¹ atoms/cm³ or lower (preferably 5×10²⁰ atoms/cm³ or lower). Inaddition, the nitrogen concentration in the insulating layer 320 ispreferably 1×10¹⁹ atoms/cm³ or lower. Note that the above-describedconcentrations mean average values in the insulating layer 320.

As a more specific example of the insulating layer 320 which fulfillsthe above-described conditions, silicon oxide formed by a sputteringmethod can be given. This is because the hydrogen concentration in thefilm can be easily reduced in a sputtering method. Needless to say, anyof other methods including a plasma CVD method may be employed as longas the above conditions are fulfilled. The other conditions of theinsulating layer 320 are not particularly limited. For example, thethickness of the insulating layer 320 can vary within a feasible range.

Next, a transparent conductive layer 322 which is electrically connectedto the drain electrode 312, a transparent conductive layer 324 which iselectrically connected to the connection electrode 314, and atransparent conductive layer 326 which is electrically connected to thesecond terminal 316 are formed (see FIG. 9C and FIG. 10).

The transparent conductive layer 322 functions as a pixel electrode, andthe transparent conductive layers 324 and 326 function as an electrodeor a wiring used for connection with flexible printed circuits (FPCs).More specifically, the transparent conductive layer 324 formed over theconnection electrode 314 can be used as a terminal electrode forconnection which functions as an input terminal of a gate wiring, andthe transparent conductive layer 326 formed over the second terminal 316can be used as a terminal electrode for connection which functions as aninput terminal of a source wiring.

In addition, a storage capacitor can be formed using the capacitorwiring 304, the gate insulating layer 308, and the transparentconductive layer 322.

The transparent conductive layers 322, 324, and 326 can be formed usinga material such as indium oxide (In₂O₃), indium tin oxide (In₂O₃—SnO₂,also abbreviated as ITO), an indium-oxide zinc-oxide alloy (In₂O₃—ZnO),or the like. For example, the transparent conductive layers 322, 324,and 326 can be formed by a sputtering method, a vacuum evaporationmethod, or the like in combination with an etching method.

In addition, after the conductive layers 322, 324, and 326 are formed,heat treatment is performed at a temperature higher than or equal to100° C. and lower than or equal to 500° C., typically higher than orequal to 200° C. and lower than or equal to 400° C. The atmosphere inwhich the heat treatment is performed can be, for example, an airatmosphere, a nitrogen atmosphere, an oxygen atmosphere, or the like.Further, the heat treatment time can be approximately more than or equalto 0.1 hours and less than or equal to 5 hours. Here, the heat treatmentis performed at 350° C. in an air atmosphere for one hour. Note that thetiming of the heat treatment is not particularly limited as long as itis after the insulating layer 320 is formed. For example, theabove-described heat treatment may be performed just after theinsulating layer 320 is formed. Alternatively, the above-described heattreatment may be performed after the contact holes are formed in theinsulating layer 320. Further alternatively, the above-described heattreatment may be performed after another insulating layer, conductivelayer, or the like is formed. By performing the heat treatment (thesecond heat treatment) and the preceding heat treatment (the first heattreatment), characteristics of the semiconductor element can be improvedand variation in characteristics can be suppressed.

Note that the effect of the second heat treatment is not limited to theabove. For example, the second heat treatment also provides anadvantageous effect of repairing defects in the insulating layer 320.Since the insulating layer 320 is formed at a relatively lowtemperature, the film includes defects. Accordingly, the elementcharacteristics might be adversely affected when the insulating layer isused as it is. From a perspective of repairing such defects in theinsulating layer 320, it can be said that the above-described heattreatment plays an important role.

Note that it is preferable that the heat treatment be performed at 400°C. or lower so as not to change (deteriorate) characteristics of thegate insulating layer 308. Needless to say, an embodiment of theinvention disclosed herein should not be interpreted as being limitedthereto.

Through the above-described process, an active matrix substrateincluding a bottom-gate transistor 350 and an element such as a storagecapacitor can be completed. For example, in the case of manufacturing anactive matrix liquid crystal display device by using this, a liquidcrystal layer may be provided between an active matrix substrate and acounter substrate provided with a counter electrode, and the activematrix substrate and the counter substrate may be fixed to each other.

Since the active matrix substrate described in this embodiment includesa transistor in which the oxide semiconductor layer 318 functions as achannel, a photosensor according to an embodiment of the presentinvention and a current amplifier can be manufactured in almost the sameprocess. In such a case, a channel of a transistor used as a photosensorand a channel of a transistor used as an oxide semiconductor elementother than the photosensor are formed in the same process; therefore,the channels are regarded as including the same material in thisspecification. A transistor utilizing an oxide semiconductor having arelatively high mobility, a small S value, and a small off-state currentcan form a photosensor according to an embodiment of the presentinvention; therefore, a multifunction semiconductor device can beobtained through a smaller number of steps. Since the refreshingoperation of the photosensor is unnecessary in the case where thevoltage is applied in a pulsed manner, it is possible to measure theilluminance of light with small power consumption through a high-speedand easy measurement procedure.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 5

In this embodiment, an example is described in which a thin filmtransistor is manufactured and a semiconductor device having a displayfunction (also referred to as a display device) is manufactured usingthe thin film transistor in a pixel portion and in a driver circuit.Further, part or whole of a driver circuit can be formed over the samesubstrate as a pixel portion, whereby a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement), a light-emitting element (also referred to as a light-emittingdisplay element), or the like can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or a voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Further, a display medium whose contrast is changed by anelectric effect, such as electronic ink, may be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Furthermore, an element substratewhich forms a display device is provided with a unit for supplyingcurrent to the display element in each pixel. Specifically, the elementsubstrate may be in a state after only a pixel electrode of the displayelement is formed, or a state after a conductive layer to be a pixelelectrode is formed and before the conductive layer is etched.

Note that a display device in this specification means an image displaydevice, a display device, a light source (including a lighting device),and the like. Further, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC(flexible printed circuit), a TAB (tape automated bonding) tape, or aTCP (tape carrier package) is attached; a module in which the tip of theTAB tape or the TCP is provided with a printed wiring board; a module inwhich an IC (integrated circuit) is directly mounted on a displayelement by a COG (chip on glass) method, and the like.

Hereinafter, in this embodiment, an example of a liquid crystal displaydevice is described. FIGS. 11A1, 11A2, and 11B are plan views and across-sectional view of a panel in which thin film transistors 4010 and4011 and a liquid crystal element 4013 which are formed over a firstsubstrate 4001 are sealed by a second substrate 4006 and a sealant 4005.Here, FIGS. 11A1 and 11A2 are each a plan view and FIG. 11B is across-sectional view taken along line M-N of FIGS. 11A1 and 11A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 that are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. In other words, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. Further, a signal linedriver circuit 4003 that is formed using a single crystal semiconductoror a polycrystalline semiconductor over a substrate separately preparedis mounted in a region different from the region surrounded by thesealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used as appropriate.FIG. 11A1 illustrates an example of mounting the signal line drivercircuit 4003 by a COG method, and FIG. 11A2 illustrates an example ofmounting the signal line driver circuit 4003 by a TAB method.

In addition, the pixel portion 4002 and the scan line driver circuit4004 provided over the first substrate 4001 each include a plurality ofthin film transistors. FIG. 11B illustrates the thin film transistor4010 included in the pixel portion 4002 and the thin film transistor4011 included in the scan line driver circuit 4004. An insulating layer4020 and an insulating layer 4021 are provided over the thin filmtransistors 4010 and 4011.

The transistors described in any of the previous embodiments or the likecan be applied to the thin film transistors 4010 and 4011. Note that inthis embodiment, the thin film transistors 4010 and 4011 are n-channelthin film transistors. Transistors similar to these transistors can beutilized in a photosensor according to an embodiment of the presentinvention and a current amplifier. Thus, it is possible to manufacture adisplay device including an oxide semiconductor and a photosensorincluding an oxide semiconductor over one substrate through almost thesame process.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. The liquid crystal element 4013 is formed by thepixel electrode layer 4030, the counter electrode layer 4031, and theliquid crystal layer 4008. Note that the pixel electrode layer 4030 andthe counter electrode layer 4031 are provided with an insulating layer4032 and an insulating layer 4033, respectively, each of which functionsas an alignment film. The liquid crystal layer 4008 is sandwichedbetween the pixel electrode layer 4030 and the counter electrode layer4031 with the insulating layers 4032 and 4033 interposed therebetween.

Note that as the first substrate 4001 and the second substrate 4006,glass, metal (typically, stainless steel), ceramic, plastic, or the likecan be used. As plastic, an FRP (fiberglass-reinforced plastics)substrate, a PVF (polyvinyl fluoride) film, a polyester film, an acrylicresin film, or the like can be used. In addition, a sheet with astructure in which an aluminum foil is sandwiched between PVF films orpolyester films can be used.

A spacer 4035 is provided in order to control the distance (a cell gap)between the pixel electrode layer 4030 and the counter electrode layer4031. For example, the spacer 4035 can be obtained by selectivelyetching an insulating layer. Note that the spacer 4035 may have any ofvarious shapes such as a columnar shape or a spherical shape. Inaddition, the counter electrode layer 4031 is electrically connected toa common potential line formed over the same substrate as the thin filmtransistor 4010. For example, the counter electrode layer 4031 can beelectrically connected to the common potential line through conductiveparticles provided between the pair of substrates. Note that theconductive particles are preferably contained in the sealant 4005.

In the case of employing a horizontal electric field mode, a liquidcrystal exhibiting a blue phase for which an alignment film isunnecessary may be used. A blue phase is one of the liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase when the temperature of cholesteric liquid crystal isincreased. Since the blue phase is only generated within a narrow rangeof temperatures, a liquid crystal composition containing a chiral agentat 5 wt % or more is preferably used. Thus, the temperature range inwhich the blue phase is exhibited can be widened. The liquid crystalcomposition which includes a liquid crystal exhibiting a blue phase anda chiral agent has a small response time of 10 μs to 100 μs, has opticalisotropy, which makes the alignment process unneeded, and has a smallviewing angle dependence.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, the present invention is not limitedthereto. An embodiment of the present invention may also be applied to areflective liquid crystal display device or a semi-transmissive liquidcrystal display device.

In this embodiment, an example of liquid crystal display device isdescribed in which a polarizing plate is provided on the outer surfaceof the substrate (on the viewer side) and a coloring layer and anelectrode layer used for a display element are provided on the innersurface of the substrate in this order (see FIG. 12); however, thepolarizing plate may be provided on the inner surface of the substrate.In addition, the stacked structure of the polarizing plate and thecoloring layer is not limited to this embodiment. The stacked structurecan be varied as appropriate in accordance with the material,manufacturing conditions, or the like of the polarizing plate and thecoloring layer. Further, a light-blocking layer which functions as ablack matrix may be provided.

In this embodiment, in order to suppress the surface roughness of thethin film transistors, the thin film transistors obtained in any of theprevious embodiments are covered with the insulating layer 4021. As theinsulating layer 4021, an organic material having heat resistance suchas polyimide, acrylic resin, benzocyclobutene resin, polyamide, or epoxyresin can be used. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of these materials.

Here, a siloxane-based resin is a resin formed using a siloxane-basedmaterial and including a bond of Si—O—Si. As a substituent, an organicgroup (e.g., an alkyl group or an aryl group) or a fluoro group may beused. In addition, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, an SOG method, a spincoating method, a dipping method, a spray coating method, or a dropletdischarge method (e.g., an ink jet method, screen printing, offsetprinting), or a tool such as a doctor knife, a roll coater, a curtaincoater, a knife coater, or the like.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed of a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a conductive composition containing a conductive highmolecule (also referred to as a conductive polymer). The pixel electrodemade of the conductive composition preferably has a sheet resistance of1.0×10⁴ Ω/sq. or less and a transmittance of 70% or more at a wavelengthof 550 nm. Furthermore, the resistivity of the conductive high moleculecontained in the conductive composition is preferably 0.1 Ω·cm or less.

As the conductive high molecule, a π-electron conjugated conductive highmolecule can be used for example. Specifically, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more of aniline, pyrrole,and thiophene or a derivative thereof, or the like can be given.

A variety of signals are supplied to the signal line driver circuit4003, the scan line driver circuit 4004, the pixel portion 4002, or thelike from an FPC 4018.

In addition, a connection terminal electrode 4015 is formed from thesame conductive layer as the pixel electrode layer 4030 included in theliquid crystal element 4013, and a terminal electrode 4016 is formedfrom the same conductive layer as source and drain electrode layers ofthe thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductivelayer 4019.

Note that FIGS. 11A1, 11A2 and 11B illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

FIG. 12 illustrates an example in which a liquid crystal display modulecorresponding to an embodiment of a semiconductor device is formed usinga substrate 2600 over which an oxide semiconductor element is formed.

In FIG. 12, the substrate 2600 over which an oxide semiconductor elementis formed and a counter substrate 2601 are bonded to each other by asealant 2602 and an element layer 2603 including an oxide semiconductorelement and the like, a liquid crystal layer 2604 including an alignmentfilm and a liquid crystal layer, a coloring layer 2605, a polarizingplate 2606, and the like are provided between the substrate 2600 and thecounter substrate 2601, whereby a display region is formed. The coloringlayer 2605 is necessary to perform color display. In the case of the RGBsystem, coloring layers corresponding to colors of red, green, and blueare provided for pixels. Polarizing plates 2606 and 2607 and a diffusionplate 2613 are provided outside the substrate 2600 over which an oxidesemiconductor element is formed and the counter substrate 2601. A lightsource includes a cold cathode tube 2610 and a reflective plate 2611. Acircuit board 2612 is connected to a wiring circuit portion 2608 of thesubstrate 2600 over which an oxide semiconductor element is formedthrough a flexible wiring board 2609. Thus, an external circuit such asa control circuit or a power source circuit is included in a liquidcrystal module. A retardation plate may be provided between thepolarizing plate and the liquid crystal layer.

For a driving method of a liquid crystal, a TN (twisted nematic) mode,an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode,an MVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above-described process, a high-performance liquid crystaldisplay device provided with a photosensor can be manufactured. Sincethe liquid crystal display device includes a transistor in which anoxide semiconductor layer functions as a channel, a photosensoraccording to an embodiment of the present invention and a currentamplifier can be manufactured in almost the same process. In such acase, a channel of a transistor used as a photosensor and the channel ofthe transistor used as an oxide semiconductor element other than thephotosensor are formed in the same process; therefore, the channels areregarded as including the same material in this specification. Atransistor utilizing an oxide semiconductor having a relatively highmobility, a small S value, and a small off-state current can form aphotosensor according to an embodiment of the present invention;therefore, a multifunction semiconductor device can be obtained througha small number of steps. Since the refreshing operation of thephotosensor is unnecessary in the case where the voltage is applied in apulsed manner, it is possible to measure the illuminance of light withsmall power consumption through a high-speed and easy measurementprocedure. This enables a rapid change of external light to be detectedby the photosensor; accordingly, the luminance of the display device canbe adjusted speedily and smoothly.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 6

In this embodiment, active matrix electronic paper which is an exampleof a semiconductor device will be described with reference to FIG. 13. Athin film transistor 650 used for the semiconductor device can bemanufactured in a manner similar to that of the transistor or the likedescribed in any of the previous embodiments. A transistor similar tothe transistor 650 can be used to manufacture a photosensor according toan embodiment of the present invention; accordingly, electronic paperwhose display state is changed in response to light can be obtained.

The electronic paper in FIG. 13 is an example of electronic paper usinga twisting ball display system. The twisting ball display system refersto a method for performing display in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer, and a potential difference is generatedbetween the first electrode layer and the second electrode layer tocontrol orientation of the spherical particles.

A source or drain electrode layer of the thin film transistor 650provided over a substrate 690 is electrically connected to a firstelectrode 660 through a contact hole formed in an insulating layer 685.A substrate 691 is provided with a second electrode 670. Between thefirst electrode 660 and the second electrode 670, spherical particles680 each having a black region 680 a and a white region 680 b areprovided. A space around the spherical particles 680 is filled with afiller 682 such as a resin (see FIG. 13). In FIG. 13, the firstelectrode 660 corresponds to a pixel electrode, and the second electrode670 corresponds to a common electrode. The second electrode 670 iselectrically connected to a common potential line provided over the samesubstrate as the thin film transistor 650.

Instead of the twisting ball, an electrophoretic display element canalso be used. In that case, for example, a microcapsule having adiameter of approximately 10 μm to 200 μm in which transparent liquid,positively-charged white microparticles, and negatively-charged blackmicroparticles are encapsulated, is preferably used. When an electricfield is applied between the first electrode and the second electrode,the white microparticles and the black microparticles move to oppositedirections from each other, so that white or black is displayed. Theelectrophoretic display element has higher reflectance than a liquidcrystal display element, and thus, an auxiliary light is unnecessary anda display portion can be recognized in a place where brightness is notsufficient. In addition, there is an advantage that even when power isnot supplied to the display portion, an image which has been displayedonce can be maintained.

Through the above-described process, high-performance electronic papercan be manufactured. Since the electronic paper includes a transistor inwhich an oxide semiconductor layer functions as a channel, a photosensoraccording to an embodiment of the present invention and a currentamplifier can be manufactured in almost the same process. In such acase, a channel of a transistor used as the photosensor and the channelof the transistor used as an oxide semiconductor element other than thephotosensor are formed in the same process; therefore, the channels areregarded as including the same material in this specification. Atransistor utilizing an oxide semiconductor having a relatively highmobility, a small S value, and a small off-state current can form aphotosensor according to an embodiment of the present invention;therefore, a multifunction semiconductor device can be obtained througha small number of steps. Since the refreshing operation of thephotosensor is unnecessary in the case where the voltage is applied in apulsed manner, it is possible to measure the illuminance of light withsmall power consumption through a high-speed and easy measurementprocedure. This enables a rapid change of external light to be detectedby the photosensor; accordingly, the display state of the electronicpaper can be changed speedily depending on external light. For example,the display of the electronic paper provided outside can beautomatically changed depending on the illuminance of external light; insuch a case, the displayed advertisement can be changed in accordancewith the weather.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 7

In this embodiment, a light-emitting display device which is an exampleof a semiconductor device will be described. Here, a case is describedin which a light-emitting element utilizing electroluminescence is usedas a display element. Note that light-emitting elements utilizingelectroluminescence are classified by whether a light-emitting materialis an organic compound or an inorganic compound. In general, the formeris called an organic EL element, and the latter is called an inorganicEL element.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound. Then, the carriers (electrons and holes) recombine, therebyemitting light. Owing to such a mechanism, the light-emitting element iscalled a current-excitation light-emitting element.

The inorganic EL elements are classified into a dispersion-typeinorganic EL element and a thin-film-type inorganic EL element dependingon the element structure. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination-type light emission which utilizes a donorlevel and an acceptor level. A thin-film-type inorganic EL element has astructure in which a light-emitting layer is sandwiched betweendielectric layers, which are further sandwiched between electrodes, andits light emission mechanism is localized-type light emission thatutilizes inner-shell electron transition of metal ions. Note that, here,description is made on an organic EL element.

Structures of the light-emitting element are described with reference toFIGS. 14A to 14C. Here, a cross-sectional structure of a pixel isdescribed by taking an n-channel driving TFT as an example. Transistors701, 711, and 721 used for semiconductor devices illustrated in FIGS.14A to 14C can be manufactured in a manner similar to that of thetransistors described in the previous embodiments. A transistor similarto these transistors can be utilized in a photosensor according to anembodiment of the present invention and a current amplifier. Thus, it ispossible to manufacture a light-emitting display device including anoxide semiconductor and a photosensor including an oxide semiconductorover one substrate through almost the same process.

In order to extract light from a light-emitting element, at least one ofthe anode and the cathode is transparent. Here, the term “beingtransparent” means having a sufficiently high transmittance to at leasta wavelength of emitted light. As a method for extracting light, in thecase where a thin film transistor and a light-emitting element areformed over a substrate, there are a top emission method (a topextraction method) in which light is extracted without passing throughthe substrate, a bottom emission method (a bottom extraction method) inwhich light is extracted through the substrate, a dual emission method(a dual extraction method) in which light is extracted from both anupper surface and a lower surface, and the like.

A top-emission-type light-emitting element will be described withreference to FIG. 14A.

FIG. 14A is a cross-sectional view of a pixel in the case where lightemitted from a light-emitting element 702 is extracted through an anode705. Here, a cathode 703 of the light-emitting element 702 and thetransistor 701 which is a driving transistor are electrically connectedto each other, and a light-emitting layer 704 and the anode 705 arestacked in this order over the cathode 703. As the cathode 703, aconductive layer which has a low work function and reflects light can beused. For example, a material such as Ca, Al, MgAg, or AlLi ispreferably used to form the cathode 703. The light-emitting layer 704may be formed to have either a single-layer structure or a stackedstructure. As an example of the stacked structure, an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer may be stacked in thisorder over the cathode 703; however, needless to say, it is notnecessary to form all of these layers and another structure may beemployed. The anode 705 is formed using a light-transmitting conductivematerial. For example, a light-transmitting conductive material such asindium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added may be used.

A structure in which the light-emitting layer 704 is sandwiched betweenthe cathode 703 and the anode 705 can be called the light-emittingelement 702. In the case of the pixel illustrated in FIG. 14A, lightemitted from the light-emitting element 702 is extracted through theanode 705 as indicated by an arrow.

Next, a bottom-emission-type light-emitting element will be describedwith reference to FIG. 14B.

FIG. 14B is a cross-sectional view of a pixel in the case where lightemitted from a light-emitting element 712 is extracted through a cathode713. Here, the cathode 713 of the light-emitting element 712 is formedover a light-transmitting conductive layer 717 which is electricallyconnected to the driving transistor 711, and a light-emitting layer 714and an anode 715 are stacked in this order over the cathode 713. Notethat a blocking layer 716 may be formed so as to cover the anode 715when the anode 715 has a light-transmitting property. The cathode 713can be formed using a conductive material having a low work function ina manner similar to that of the case of FIG. 14A. Note that the cathode713 is formed to such a thickness as to transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of approximately 20 nm can be used as the cathode 713. In amanner similar to that of the case of FIG. 14A, the light-emitting layer714 may be formed to have either a single-layer structure or a stackedstructure. The anode 715 is not required to transmit light, but may beformed using a light-transmitting conductive material in a mannersimilar to that of the case of FIG. 14A. As the blocking layer 716, ametal which reflects light or the like can be used; however, it is notlimited thereto. For example, a resin to which a black pigment is addedor the like can also be used.

A structure in which the light-emitting layer 714 is sandwiched betweenthe cathode 713 and the anode 715 can be called the light-emittingelement 712. In the case of the pixel illustrated in FIG. 14B, lightemitted from the light-emitting element 712 is extracted from thecathode 713 as indicated by an arrow.

Next, a dual-emission-type light-emitting element will be described withreference to FIG. 14C.

In FIG. 14C, a cathode 723 of a light-emitting element 722 is formedover a light-transmitting conductive layer 727 which is electricallyconnected to the driving transistor 721, and a light-emitting layer 724and an anode 725 are stacked in this order over the cathode 723. Thecathode 723 can be formed using a conductive material having a low workfunction in a manner similar to that of the case of FIG. 14A. Note thatthe cathode 723 is formed to such a thickness as to transmit light. Forexample, an aluminum film with a thickness of 20 nm can be used as thecathode 723. In a manner similarly to that of the case of FIG. 14A, thelight-emitting layer 724 may have either a single-layer structure or astacked structure. In a manner similar to that of the case of FIG. 14A,the anode 725 can be formed using a light-transmitting conductivematerial.

A structure where the cathode 723, the light-emitting layer 724, and theanode 725 overlap with one another can be called the light-emittingelement 722. In the case of the pixel illustrated in FIG. 14C, lightemitted from the light-emitting element 722 is extracted through boththe anode 725 and the cathode 723 as indicated by arrows.

Although a case of using an organic EL element as a light-emittingelement is described here, an inorganic EL element can also be used as alight-emitting element. The example is described here in which a thinfilm transistor (a driving transistor) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a transistor for current control or the like may beconnected between the driving transistor and the light-emitting element.

Note that the structure of the semiconductor device described in thisembodiment is not limited to those illustrated in FIGS. 14A to 14C andcan be modified in various ways.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel) will be describedwith reference to FIGS. 15A and 15B. FIGS. 15A and 15B are a plan viewand a cross-sectional view of a panel in which thin film transistors4509 and 4510 and a light-emitting element 4511 which are formed over afirst substrate 4501 are sealed by a second substrate 4506 and a sealant4505. FIG. 15A is a plan view and FIG. 15B is a cross-sectional viewtaken along line H-I in FIG. 15A.

The sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b, which are provided over the first substrate4501. In addition, the second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. In other words, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. Packaging (sealing) is preferably performed usinga protective film (such as a bonding film or an ultraviolet curableresin film), a cover material, or the like with high air-tightness andlittle degasification.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b, which are formedover the first substrate 4501, each include a plurality of thin filmtransistors. In FIG. 15B, the thin film transistor 4510 included in thepixel portion 4502 and the thin film transistor 4509 included in thesignal line driver circuit 4503 a are illustrated as an example.

As the thin film transistors 4509 and 4510, any of the transistorsdescribed in the previous embodiments can be employed. Note that in thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors. A transistor similar to these transistors can beutilized in a photosensor according to an embodiment of the presentinvention and a current amplifier. Thus, it is possible to manufacture alight-emitting display device including an oxide semiconductor and aphotosensor including an oxide semiconductor over one substrate throughalmost the same process.

Moreover, a first electrode 4517 that is a pixel electrode of thelight-emitting element 4511 is electrically connected to a sourceelectrode or a drain electrode of the thin film transistor 4510. Thestructure of the light-emitting element 4511 is a stacked structure ofthe first electrode 4517, an electroluminescent layer 4512, and a secondelectrode 4513; however, it is not limited to the structure described inthis embodiment. The structure of the light-emitting element 4511 can bechanged as appropriate depending on the direction in which light isextracted from the light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin, an inorganicinsulating layer, organopolysiloxane, or the like. It is particularlypreferable that the partition 4520 be formed of a photosensitivematerial to have an opening over the first electrode 4517 so that asidewall of the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may have either a single-layerstructure or a stacked structure.

A protective film may be formed over the second electrode 4513 and thepartition 4520 in order to prevent oxygen, hydrogen, moisture, carbondioxide, or the like from entering the light-emitting element 4511. Theprotective film can be formed using silicon nitride, silicon nitrideoxide, DLC (diamond like carbon), or the like.

A variety of signals are supplied to the signal line driver circuits4503 a and 4503 b, the scan line driver circuits 4504 a and 4504 b, thepixel portion 4502, or the like from FPCs 4518 a and 4518 b.

In this embodiment, an example is described in which a connectionterminal electrode 4515 is formed from the same conductive layer as thefirst electrode 4517 of the light-emitting element 4511, and a terminalelectrode 4516 is formed from the same conductive layer as the sourceand drain electrodes of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive layer 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty. As a substrate having a light-transmitting property, a glassplate, a plastic plate, a polyester film, an acrylic film, and the likeare given.

As the filler 4507, an ultraviolet curable resin, a thermosetting resin,or the like can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinylacetate (EVA), or the like can be used. In this embodiment, an examplein which nitrogen is used for the filler is described.

If needed, an optical film, such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter, may be provided on a surface on the light-emission side ofthe light-emitting element. Furthermore, antireflection treatment may beperformed on a surface of the light-emitting element. For example,anti-glare treatment by which reflected light can be diffused byprojections and depressions on the surface can be performed forsuppressing the glare.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared. Alternatively, only the signal line driver circuitsor part thereof, or only the scan line driver circuits or part thereofmay be separately formed and mounted. This embodiment is not limited tothe structure illustrated in FIGS. 15A and 15B.

Through the above-described process, a high-performance light-emittingdisplay device (display panel) provided with a photosensor can bemanufactured. Since the light-emitting display device includes atransistor in which an oxide semiconductor layer functions as a channel,a photosensor according to an embodiment of the present invention and acurrent amplifier can be manufactured in almost the same process. Insuch a case, a channel of a transistor used as the photosensor and thechannel of the transistor used as an oxide semiconductor element otherthan the photosensor are formed in the same process; therefore, thechannels are regarded as including the same material in thisspecification. A transistor utilizing an oxide semiconductor having arelatively high mobility, a small S value, and a small off-state currentcan form a photosensor according to an embodiment of the presentinvention; therefore, a multifunction semiconductor device can beobtained through a small number of steps. Since the refreshing operationof the photosensor is unnecessary in the case where the voltage isapplied in a pulsed manner, it is possible to measure the illuminance oflight with small power consumption through a high-speed and easymeasurement procedure. This enables a rapid change of external light tobe detected by the photosensor; accordingly, the luminance of thedisplay device can be adjusted speedily and smoothly.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 8

An example of electronic paper provided with a photosensor according toan embodiment of the present invention will be described. Electronicpaper can be used for electronic devices of a variety of fields. Forexample, electronic paper can be applied to an electronic book reader(e-book), a poster, an advertisement in a vehicle such as a train,displays of various cards such as a credit card, and the like. Examplesof the electronic devices are illustrated in FIGS. 16A and 16B and FIG.17.

FIG. 16A illustrates a poster 2631 formed using electronic paperprovided with a photosensor 2630 according to an embodiment of thepresent invention. In the case where an advertising medium is printedpaper, the advertisement is replaced by manpower; however, by usingelectronic paper, the advertising display can be changed in a shorttime. Further, an image can be stably displayed without displaydeterioration. Since the photosensor 2630 is provided, the display statecan be changed depending on the illuminance of external light. Note thatthe poster may have a configuration capable of wirelessly transmittingand receiving data.

FIG. 16B illustrates an advertisement 2632 provided with a photosensor2633 according to an embodiment of the present invention, in a vehiclesuch as a train. In the case where an advertising medium is printedpaper, the advertisement is replaced by manpower; however, by usingelectronic paper, the advertising display can be changed in a short timewithout a lot of manpower. Further an image can be stably displayedwithout display deterioration. Since the photosensor 2633 is provided,the display state can be changed depending on the illuminance ofexternal light. Note that the advertisement in a vehicle may have aconfiguration capable of wirelessly transmitting and receiving data.

FIG. 17 illustrates an example of an electronic book reader providedwith a photosensor 2730 according to an embodiment of the presentinvention. For example, an electronic book reader 2700 includes twohousings, a housing 2701 and a housing 2703. The housing 2701 and thehousing 2703 are combined with a hinge 2711 so that the electronic bookreader 2700 can be opened and closed using the hinge 2711 as an axis.With such a structure, the electronic book reader 2700 can be operatedlike a paper book. Since the photosensor 2730 is provided, the displaycan be automatically turned on or off in response to the opening andclosing operation of the electronic book reader 2700, for example.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right side (the display portion 2705 in FIG. 17) can display textand a display portion on the left side (the display portion 2707 in FIG.17) can display graphics.

FIG. 17 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Further, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to various cables such as an AC adapterand a USB cable, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Further, the electronic book reader 2700 may have a function ofan electronic dictionary.

The electronic book reader 2700 may have a configuration capable ofwirelessly transmitting and receiving data. A structure may be employedin which a desired book data or the like is purchased and downloadedfrom an electronic book server wirelessly.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Embodiment 9

A photosensor according to an embodiment of the present invention can bemounted on a variety of electronic devices (including game machines).Examples of such electronic devices are a television device (alsoreferred to as a television or a television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone (also referred to asa cellular phone or a mobile phone device), a portable game console, aportable information terminal, an audio playback device, a large-sizedgame machine such as a pinball machine, and the like.

FIG. 18A illustrates an example of a television device provided with aphotosensor 9611 according to an embodiment of the present invention. Ina television device 9600, a display portion 9603 is incorporated in ahousing 9601. The display portion 9603 can display images. Here, thehousing 9601 is supported by a stand 9605.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote controller 9610. Channels can beswitched and the volume can be controlled with operation keys 9609 ofthe remote controller 9610, whereby an image displayed on the displayportion 9603 can be controlled. Moreover, the remote controller 9610 maybe provided with a display portion 9607 for displaying data output fromthe remote controller 9610.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general TV broadcasts can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (e.g., between a sender and areceiver or between receivers) information communication can beperformed.

FIG. 18B illustrates an example of a digital photo frame provided with aphotosensor 9705 according to an embodiment of the present invention.For example, in a digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displayimage data taken with a digital camera or the like and function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalconnectable to a variety of cables such as a USB cable), a recordingmedium insertion portion, and the like. Although these components may beprovided on the same surface as the display portion, it is preferable toprovide them on the side surface or the back surface for designaesthetics. For example, a memory that stores image data taken with adigital camera is inserted into the recording medium insertion portionof the digital photo frame 9700 and the data is loaded, whereby theimage can be displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. Through wireless communication, desired image data canbe loaded to be displayed.

FIG. 19A illustrates a portable game console including a housing 9881and a housing 9891 which are jointed with a joint portion 9893 so thatthe portable game console can be opened or folded. A display portion9882 and a display portion 9883 are incorporated in the housing 9881 andthe housing 9891, respectively. In addition, the portable game consoleillustrated in FIG. 19A is provided with a speaker portion 9884, arecording medium insertion portion 9886, an LED lamp 9890, input means(operation keys 9885, a connection terminal 9887, a sensor 9888 (onehaving a function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), and amicrophone 9889), and the like. Needless to say, the structure of theportable game console is not limited to the above and another structurewhich is provided with at least a semiconductor device can be employed.The portable game console may include an additional accessory asappropriate. The portable game console illustrated in FIG. 19A has afunction of reading a program or data stored in a recording medium todisplay it on the display portion, and a function of sharing data withanother portable game console via wireless communication. Note that afunction of the portable game console illustrated in FIG. 19A is notlimited to those described above, and the portable game console can havea variety of functions.

FIG. 19B illustrates an example of a slot machine which is a large-sizedgame machine provided with a photosensor 9905 according to an embodimentof the present invention. In a slot machine 9900, a display portion 9903is incorporated in a housing 9901. In addition, the slot machine 9900includes an operation means such as a start lever or a stop switch, acoin slot, a speaker, and the like. Needless to say, the structure ofthe slot machine 9900 is not limited to the above and another structurewhich is provided with at least a semiconductor device may be employed.The slot machine 9900 may include an additional accessory asappropriate.

FIG. 20A illustrates an example of a mobile phone provided with aphotosensor 1007 according to an embodiment of the present invention. Amobile phone 1000 includes a housing 1001 in which a display portion1002 is incorporated, an operation button 1003, an external connectionport 1004, a speaker 1005, a microphone 1006, and the like.

Information can be input to the mobile phone 1000 illustrated in FIG.20A by touching the display portion 1002 with a finger or the like.Moreover, users can make a call or compose a mail by touching thedisplay portion 1002 with their fingers or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting information such as text. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or composing a mail, thedisplay portion 1002 may be placed into a text input mode mainly forinputting text, and characters displayed on a screen can be input. Inthis case, it is preferable to display a keyboard or number buttons onalmost the entire area of the screen of the display portion 1002.

When a detection device including a sensor which detects inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display on the screen of the display portion 1002 canbe automatically switched by detecting the direction of the mobile phone1000 (whether the mobile phone 1000 is placed horizontally or verticallyfor a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion1002 or operating the operation button 1003 of the housing 1001.Alternatively, the screen modes can be switched depending on the kindsof image displayed on the display portion 1002. For example, when asignal for an image displayed on the display portion is data of movingimages, the screen mode is switched to the display mode. When the signalis text data, the screen mode is switched to the input mode.

Further, in the input mode, a signal is detected by a photosensorincorporated in the display portion 1002 and if input by touching thedisplay portion 1002 is not performed for a certain period, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode. As the photosensor, any of the photosensors described inthe above embodiments can be used.

The display portion 1002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenwhen the display portion 1002 is touched with the palm or the finger,whereby personal authentication can be performed. Moreover, when abacklight or a sensing light source which emits near-infrared light isprovided in the display portion, an image of finger veins, palm veins,or the like can be taken.

FIG. 20B illustrates another example of a mobile phone provided with aphotosensor 9415 according to an embodiment of the present invention.The mobile phone in FIG. 20B has a display device 9410 provided with ahousing 9411 including a display portion 9412 and operation buttons9413, and a communication device 9400 provided with a housing 9401including operation buttons 9402, an external input terminal 9403, amicrophone 9404, a speaker 9405, and a light-emitting portion 9406 thatemits light when a phone call is received. The display device 9410having a display function can be detachably attached to thecommunication device 9400 having a phone function in two directionsrepresented by the arrows. Thus, the display device 9410 and thecommunication device 9400 can be attached to each other along theirshort sides or long sides. In addition, when only the display functionis needed, the display device 9410 can be detached from thecommunication device 9400 and used alone.

Images or input information can be transmitted or received by wirelessor wire communication between the communication device 9400 and thedisplay device 9410, each of which has a rechargeable battery.

Since the photosensor according to an embodiment of the presentinvention is provided in each of the above-described devices, the stateof the display portion can be automatically changed in response to achange of external light or the opening and closing operation of thedevice. This embodiment can be implemented in combination with any ofthe other embodiments, as appropriate.

This application is based on Japanese Patent Application serial no.2010-138916 filed with Japan Patent Office on Jun. 18, 2010, the entirecontents of which are hereby incorporated by reference.

1. A light measurement method comprising the steps of: applying anegative gate voltage to a transistor including a channel including anoxide semiconductor in a pulsed manner; and measuring an illuminance oflight received by the channel from an obtained gate current.
 2. Thelight measurement method according to claim 1, wherein the gate voltageis higher than or equal to −10 V and lower than or equal to −2 V.
 3. Thelight measurement method according to claim 1, wherein the gate voltageis set to 0 and a voltage of higher than or equal to 2 V and lower thanor equal to 10 V is applied to a source electrode and a drain electrodeof the transistor.
 4. The light measurement method according to claim 1,wherein the gate voltage is applied for a period of more than or equalto 1 ms and less than or equal to 2 ms.
 5. The light measurement methodaccording to claim 1, wherein the number of applications of the gatevoltage per unit time is more than or equal to 30 times per minute andless than or equal to 60 times per minute.
 6. A photosensor comprising:a transistor including a channel including an oxide semiconductor; andan oscillator circuit, wherein an output of the oscillator circuit iselectrically connected to a gate electrode of the transistor, andwherein the channel is a light receiving portion.
 7. A semiconductordevice comprising: the photosensor according to claim 6; and an RFIDdevice that operates using a transistor including a channel includingthe same material as the channel of the transistor included in thephotosensor.
 8. A semiconductor device comprising: the photosensoraccording to claim 6; and a display device that operates using atransistor including a channel including the same material as thechannel of the transistor included in the photosensor.
 9. Asemiconductor device comprising: the photosensor according to claim 6;and electronic paper that operates using a transistor including achannel including the same material as the channel of the transistorincluded in the photosensor.
 10. A photosensor comprising: a transistorincluding a channel including an oxide semiconductor; and an oscillatorcircuit, wherein an output of the oscillator circuit is electricallyconnected to a source electrode and a drain electrode of the transistor,and wherein the channel is a light receiving portion.
 11. Asemiconductor device comprising: the photosensor according to claim 10;and an RFID device that operates using a transistor including a channelincluding the same material as the channel of the transistor included inthe photosensor.
 12. A semiconductor device comprising: the photosensoraccording to claim 10; and a display device that operates using atransistor including a channel including the same material as thechannel of the transistor included in the photosensor.
 13. Asemiconductor device comprising: the photosensor according to claim 10;and electronic paper that operates using a transistor including achannel including the same material as the channel of the transistorincluded in the photosensor.